1. Field of Invention
This invention relates in general to electrical signal filters and, in particular, to an integrated circuit filter arrangement whose filter characteristics are adjustable.
2. Cross-Referenced Applications
Ser. No. 494,822, filed concurrently herewith in the name of Harr and Hense, entitled "Recording Channel with Signal Controlled Integrated Analog Circuit" and common assigned, discloses and claims a disk file recording channel in which the present invention is advantageously employed.
3. Description of the Prior Art
The cross-referenced application discloses a recording channel for a disk file which requires a filter that is adjustable. The read portion of the recording channel on disk files employs a filter arrangement for eliminating unwanted noise and for shaping the frequency and pulse characteristics of the signal from the magnetic read head. The cut-off frequency of such a filter is designed to be slightly higher than the highest frequency in the readback signal so that as much unwanted noise as possible is rejected. Also, particular frequencies in the readback signal are sometimes emphasized by the filters to enhance the performance of the signal processing circuits downstream, such as the detector.
Historically, these filters are made of discrete passive components, i.e., inductors and capacitors. The filter characteristics are, therefore, fixed at design time by the parameters of these passive components. This prevents a single filter from obtaining optimum performance if, for example, the disk file has a banded recording type of technique for storing data. In banded recording, the outer tracks on the disk surface have a much higher number of bits per track than inner tracks, which results in a higher readback frequency on the outer tracks. If a single filter is to be used in such an environment, then the filter cut-off frequency must be set high enough to properly pass the signals at the outer band. However, on the inner band where the readback signal is much lower, the filter cut-off frequency is now set too high to eliminate unwanted noise above the lower frequency. Thus, excessive noise exists when reading the inner band of tracks. Also, the frequencies that were emphasized in the outer band now have been shifted at the inner band and are not emphasized properly.
The prior art has suggested various arrangements for adjusting filters, none of which have proved applicable for use in a filter for a recording channel of a disk file. See, for example, section 8 of the article entitled "Monolithic Filters" beginning on page 83 of the Digest of Technical Papers of the 1980 IEEE International Solid State Circuit Conference.
The sampling or switching techniques discussed on pages 88-97 of that referenced digest are not applicable because the filters have bandwidths that are too low to be useful in a recording channel. The gyrator apparatus described on pages 86-87 of that reference is operable at low frequencies, but designing an active gyrator circuit to operate in the range of typical present day recording channels, i.e., 10 through 50 MHz, becomes very difficult, if not impossible.
The prior art has also disclosed generally the implementation of filters employing integrated circuits, but, as disclosed in the cross-referenced application, these filters rely on the use of external components to obtain, the final adjustment of their characteristics. As discussed in that application, this becomes quite costly from a manufacturing standpoint and actually adversely affects the operation of the integrated circuit.
Further, while the art teaches generally adjusting the natural frequency and the damping characteristics of filters, in the disclosed prior art arrangement these adjustments are not mutually independent of each other in that an adjustment of the natural frequency, for example, affects the damping characteristics of the filter and vice versa. A filter arrangement in which the natural frequency may be adjusted independently of the damping characteristics and which is implemented in integrated circuit technologies without any external components would be advantageous for a recording channel of a disk file and several other filter applications. The present invention provides such a filter arrangement.